Dressing refers to an abrasive operation frequently used in fabricating new or reconditioning used abrasive tools, i.e., grinding or cutting tools. These tools typically have a structurally supportive core and an abrasive portion of discrete abrasive grains held to the core by a binding component. A grinding wheel is a common example of such a tool. As initially produced, such tools often exhibit slight geometric irregularities, especially at the surface, that define the operative cutting edge of the tool. Also, abrasive tools routinely become dull as they are used. Dullness results largely from retention by the binding component of worn abrasive particles exposed to repeated impact with the work piece. It is also caused by a loss of exposed cutting edge as spaces between the abrasive particles are filled by abrasion debris.
The dressing operation normally involves mechanical shaping of an abrasive tool in which the dressing blade is held against or applied to the cutting edge and produces controlled abrasion of the tool. Dressing removes excess material from high spots of the abrasive portion. Manufacturers thus normally use dressing in late steps of abrasive tool fabrication to shape the cutting edge to a desired profile. Dressing also refers to making the tool dimensions conform precisely with design tolerance specifications. For example, dressing can be used on a grinding wheel in such a fashion that the cutting edge of the wheel will run true when it rotates in operation. Dressing also can sharpen and restore used tools to free cutting condition. This is done by abrasively removing bond material that has failed to erode to expose new underlying abrasive grains after outer grains have been consumed, and sculpting out work piece debris and binding component residue which accumulate between grains during primary grinding operations.
An abrasive portion of a conventional dressing tool typically contains diamond grains positioned systematically or randomly, often in a planar arrangement. The abrasive portion is joined to a base which allows fixing the tool to a machine adapted to carry out dressing. The abrasive portion is applied to the base so that the cutting edge of the dressing tool can be disposed tangentially to the abrasive tool to be dressed. Controlled abrasion is effected by the diamond grains which are located at the tip of the dressing tool and are outwardly exposed to the abrasive tool.
Wear characteristics of a dressing tool during the dressing process are a great concern for the manufacturer of abrasive tools. If the dressing tool wears rapidly, it must be replaced with high frequency. Dressing tools use costly materials such as diamond. They are made to high standards of quality and dimensional precision. Hence, the fabrication of dressing tools is usually complicated and labor intensive, and dressing tools are relatively expensive. Therefore, it is important to the manufacturer of abrasive tools to have available durable dressing tools that provide extended service life.
Wear of the diamond grains of the dressing tool is relatively minor because the abrasive portion of the tool being dressed is generally softer than the diamond. Significant wear stems from deterioration of the bonding material that joins the diamond to the base of the dressing tool. A major reason for deterioration is that the bonding material is itself worn away by contact with the work piece during dressing. The mass of bonding material embedding diamond grains diminishes during service until an insufficient amount of material remains to retain those grains. Usually a metallic bonding material is used to surround the diamond grains of dressing tools as a means to withstand the abrasive action of the grinding wheel. Preferably, the composition of the metal bond in which diamond grains are embedded is selected to provide a fairly high wear resistance.
Metal bonds of diamond grains to dressing tools are conventionally affected with compositions that include metal elements, metal compounds and alloys thereof. The metal bond composition is sometimes formed by a brazing process. Broadly summarized, this process involves heating a well dispersed mixture of fine particles of the components to a temperature at which they melt and flow around the grains. Then the tool is cooled so that the fused bond composition solidifies, embeds the grains and adheres them to the metal base of the tool. Another metal bonding technique includes compressing diamond grains and a metal powder mixture to form a compacted abrasive element of preformed shape. Heat treating the compacted abrasive element causes sintering, i.e., densifying the metal powder mixture without liquefying the entire mixture such that the diamond grains become bound by the sintered metal. This is occasionally referred to as powder metallurgy bonding technology.
Another significant factor contributing to premature release of the diamond grains from the dressing tool is strength of the metal bond. Weaker bonds will fail and release diamond grains under service conditions more quickly than stronger bonds, and thus weakly bonded tools will suffer from accelerated wear.
Diamond normally does not bond well to many metals and metal alloys that are desirable for brazed bond compositions. Techniques have been developed to increase bond strength that entail incorporating a reactive metal ingredient such as titanium, chromium or zirconium, in the precursor bond composition. This reactive metal ingredient is characterized by its ability to react directly with the diamond grain to form a strong chemical bond with the grain. These so-called “active metal” bond compositions thus have both non-reactive and reactive components. Usually the non-reactive components constitute most of the bond composition. The non-reactive components alloy to form a strong and durable bond which is adhesive to the base. The reactive component tenaciously attaches by chemical bond to the superabrasive and is cohesive with the non-reactive alloy. For example, U.S. Pat. No. 4,968,326 to Wiand discloses a method of making a diamond cutting and abrading tool which comprises mixing a carbide forming substance with a braze alloy and temporary binder, applying the mixture to a tool substrate, applying diamond particles onto the mixture coated tool and heating the thus combined materials to initially form a carbide coating on the diamond. Thereafter the carbide coated diamond is brazed to the tool. The brazing alloys disclosed are nickel, silver, gold or copper based.
It is a particularly important aspect of creating a durable dressing tool that the metal bond composition embedding the diamond grains has an adequate interface with the metal base to provide strong attachment. Geometry of the base can be an important factor. FIG. 4 of PCT Publication No. WO 00/6340 (Feb. 10, 2000) illustrates the rim construction of a rotary dressing tool in which four abrasive grains are arranged in a stack to form a single grain width cutting edge protruding from the metal core of the tool. The rim is formed to a width equal to the width of the grains so that only a narrow circumferential area of the rim is in contact with the bond material and there is no lateral support other than the structure of the inter-grain bond material. Other dressing tool configurations such as FIGS. 2 and 3 of U.S. Pat. No. 4,805,586, include a metal backing structure of the dressing tool base. This backing structure provides more area for the metal bond to adhere to the base and thus should provide a stronger connection between the metal bond and the base.
It is desirable to have a dressing tool that has superior wear resistance such that the frequency of dressing tool replacement can be reduced. It is also very much desired to provide a dressing tool that can be fabricated more simply and less laboriously than conventional tools.